1. Field of the Invention
The present invention relates to a method and related communication device and more particularly, to a method for improving discontinuous reception corresponding to hybrid automatic repeat request (HARQ) processes for a wireless communication system and related communication device.
2. Description of the Prior Art
The third generation (3G) mobile communications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA can provide high frequency spectrum utilization, universal coverage, and high quality, high speed multimedia data transmission. The WCDMA method also meets all kinds of QoS (Quality of Service) requirements simultaneously, providing diverse flexible two-way transmission services and better communication quality to reduce transmission interruption rates.
Long Term Evolution wireless communications system (LTE system), an advanced high-speed wireless communications system established upon the 3G mobile telecommunications system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B alone rather than in Node B and RNC (Radio Network Controller) respectively, so that the system structure becomes simpler.
The MAC layer of a user equipment (UE) includes a Hybrid Automatic Repeat Request (HARQ) entity which can simultaneously execute a plurality of HARQ processes for parallel data transmissions.
According to the structure of LTE system, Discontinuous Reception (DRX) is applied to the MAC layer for allowing UE to enter a standby mode during certain periods and stopping monitoring a Physical down Link Control Channel (PDCCH).
Regarding a Down Link Hybrid Automatic Repeat Request (DL HARQ) process, a Hybrid Automatic Repeat Request Round Trip Time (HARQ RTT) timer is used for providing a certain amount of consecutive Transmission Time Intervals (TTIs) during which the UE enters the standby mode. After the TTIs, the UE executes a retransmission detection of a specified DL HARQ process. In this application, HARQ RRT timer is hereinafter abbreviated as round trip time (RTT) timer for easy studying purpose.
Apart from that, a DRX retransmission timer is used for providing a certain amount of TTIs during which UE waits for a retransmission packet of the DL HARQ process. When receiving the retransmission packet during the TTI period, the UE instantly stops the DRX retransmission timer. Contrarily, the UE enters the standby mode when not receiving any retransmission packet during the TTI period.
According to the prior art, missing of a retransmission packet occurs when the UE uses only one DRX retransmission timer to manage retransmission reception timings of multiple DL HARQ processes.
Please refer to FIG. 1, which is a schematic diagram of retransmission detection timing of a DL HARQ process managed by a UE according to the prior art. The UE uses DL HARQ processes A and B to receive packets 600A and 602B, respectively. The suffixes A and B of the packet 600A and 602B represent exactly which DL HARQ process they belong to. When the UE detects the packet 600A, a RTT timer HTR1 starts running. When the RRT timer HTR1 expires at time T1, a DRX retransmission timer DRT1 is started and provides a retransmission reception time T4 for the UE. When the packet 602B is detected, the UE starts a RTT timer HTR2. When the RTT timer HTR2 expires at time T3, the UE restarts the DRX retransmission timer DRT1 because the DRX retransmission timer DRT1 is still running. When a retransmission packet 604A of the packet 600A is received at time T5, the UE stops the DRX retransmission timer DRT1 and enters the standby mode. In this situation, UE will miss a retransmission packet of the packet 602B.